The present invention relates to dynamoelectric machines and, more particularly, to electric motors and generators of the type having a DC rotor field receiving DC excitation from an AC generator which has its rotor integrally rotating with the DC rotor field and a plurality of rectifiers, also rotating with the rotor field for converting the AC from the AC generator into DC for the DC rotor field. This type of exciter is generally referred to as a brushless exciter, since DC excitation is generated on the rotating elements without requiring brushes to transfer it from a non-rotating location.
In dynamoelectric machines operating at speeds of 3600 RPM or below, hereinafter referred to as low-speed machines, the necessary rectifiers, usually six in number, are mounted on a disc which rotates with the rotor shaft. It is customary to use pigtail-type rectifiers in this application. As the shaft speed is increased to, for example 6000 RPM, several undesirable effects interfere with the use of pigtail-type rectifiers on a shaft-mounted disc. The principal problem results from the fact that, at such high rotational speeds, the shaft overhang of such a disc carrying a load of pigtail-type diodes adds resonance problems to the critical speeds of the machine.
One attempt to solve this problem is disclosed in U.S. Pat. No. 2,897,383 to Barrows et al in which pigtail-type diodes are disposed on the axis of rotation within the shaft to reduce the centrifugal forces and to reduce the overhanging mass. This structure may be satisfactory for low-speed machines, but at higher speeds, the shaft diameters must be so greatly reduced to limit the centrifugal stress on the material of the shaft that insufficient space remains within a practical shaft diameter to allow installation of pigtail-type diodes. In addition, the presence of the pigtails and necessary wire connections thereto aggravates the problems of precise balancing.
A type of small, efficient, diode exists which does not employ pigtail connections and which is axially symmetric. This type of diode is sometimes referred to as a hockey-puck type diode. The hockey-puck diode, typified by diode type A390P, manufactured by the General Electric Company, as well as equivalent types manufactured, for example, by International Rectifier Corp., is essentially a small cylindrical casing containing a mass of silicon and having planar nickel-plated copper contact electrodes at the ends of the cylindrical casing. Conventional doping of the silicon establishes the diode characteristics. Electrical connection to a hockey-puck diode is made by metallic contacts forcibly urged along the axis of the diode against the two contact electrodes. For proper operation, a hockey-puck diode requires a relatively precise contact force. For example, one type of such diode requires a contact force of within about 10 percent of 800 pounds.
Due to the need for precise contact force, hockey-puck diodes cannot be used off the axis of rotating elements of high-speed machines. Centrifugal forces in such locations would cause variations in the contact force which exceeds the specifications for efficient operation of the diodes.
All diodes carrying substantial power and operating in enclosed spaces require cooling to limit their temperature rise to values which give reasonable lifetimes. Preferably, such cooling should be performed without adding substantially to the rotating load.